Well to make it clear Venerana was telling Rias who is the next-heir of Gremory, Rias has to be in charge of things. In this case she's telling Rias to control and manage Ise's harem. Though she doesnt use the word "Harem-master" at all. She uses the word "heir" as in the heir of Gremory household(future-tense). Does Chinese translation use "Harem-master"? I mean Ise would react by saying "Did I just hear harem!?" if that happens.